1. Field of the Invention
The present invention relates generally to high-performance light emitting diodes, and more particularly to a method of using metal diffusion bonding technology as well as surface treatment process to improve light extraction efficiency.
2. Description of the Related Art
The MOCVD epitaxy technology has been applied in the production of high brightness Group III-V light emitting diodes (LEDs), which include aluminum indium gallium phosphide (AlInGaP, wavelength range: 560 nm˜650 nm) LEDs and indium gallium nitride (InGaN, wavelength range: 380 nm˜550 nm) LEDs. High brightness LEDs are employed extensively in the areas of indicating lamps, traffic lights, outdoors displays, special lighting, etc. The epitaxial growth of AlInGaP LED is usually grown on lattice-matched GaAs substrate in order to get good crystal quality. However, since the energy bandgap of GaAs substrate is less than the emission energy of LED, at least half of the emitted light will be absorbed by the light-absorbing GaAs substrate. Improved performance can be achieved by eliminating this light-absorbing GaAs substrate. There are several techniques for replacing this light-absorbing substrate by either a transparent substrate or an opaque substrate.
U.S. Pat. No. 5,376,580 issued to Fred A. Kish, et al in 1993 disclosed a semiconductor light emitting diode structure and manufacturing process as shown in FIG. 1, wherein a wafer bonding was performed at a high temperature and a high pressure (temperature >900° C., pressure >400 kg/cm2) to bond the epitaxial LED layers 61 and the transparent GaP semiconductor substrate 60. The light-absorbing GaAs substrate can be removed either before or after the wafer bonding. To reduce the resistance at the wafer-bond interface, Fred A. Kish suggested that the bonding interface must have an InGaP layer as a bonding layer 62. Such bonding method is usually called fusion bonding. The fusion bonding process has the following drawbacks: (1) after the removal of the GaAs substrate, the remaining LED structure is very thin, e.g., ˜50 μm, and therefore fragile and difficult to handle, so that it is difficult to maintain the integrity of the bonded wafer or achieve a high yield rate; and (2) it is necessary to align precisely the lattice direction of LED structure and the permanent transparent substrate during the wafer bonding process, otherwise the operating voltage of the LED will be high.
U.S. Pat. No. 6,797,987 B2 issued to Tzer-Perng Chen in 2003 disclosed a semiconductor LED structure as shown in FIG. 2. Epitaxial structure 100 was grown on a temporary light-absorbing GaAs substrate by MOCVD. Thereafter, an ohmic contact metal 112 was formed on the p-type contact layer (GaAsP, AlGaAs or GaP). Then, a transparent conducting oxide layer 114 and a metal reflective layer 116 were deposited for reflecting the light generated in the active layer 106. The transparent conducting oxide layer 114 also served the function of conducting currents, so as to provide a vertical LED chip structure. Further, a permanent substrate 120 such as Si, AlN, SiC, copper, or aluminum was bonded to epitaxial structure 100 by using the bonding layer 124 such as Au—Sn, Au—Si, Pb—Sn, Au—Ge, or In alloy. The principle is same as soldering. During the heat up, when temperature reaches the eutectic point or melting point of the metal alloy chosen, the bonding layer 124 melted. When temperature cooled down, the liquid metal alloy solidified, and bonded epitaxial structure 100 to the permanent substrate 120. Since the bonding layer 124 is melted during the pressurized bonding process, the pressure applied for the bonding process must be less than 15 kg/cm2 to prevent the melted metal from being squeezed out during the bonding process. Such bonding method is called eutectic bonding. The eutectic points or melting points of some commonly used metal bonding layer are listed below:
Typical Eutectic Point or Melting PointIn156° C.Au—20Sn276° C.Au—3.2Si363° C.Pb—61.9Sn183° C.Au—12.5Ge381° C.
Usually, the eutectic bonding is categorized as a low-temperature (Typ. <400° C.) and/or low-pressure (<15 kg/cm2) wafer bonding. This method has the following drawbacks: (1) Since this kind of eutectic reactions is fast, it is not easy to control the thickness of the metal bonding layer after bonding; (2) It is likely to produce an intermetallic compound (IMC) which is a fragile material, and the IMC further deteriorates the bonding strength; and (3) The bonded wafer can not go through a temperature higher than the bonding temperature in the chip process steps following wafer bonding, otherwise, the metal bonding layer will be affected. Furthermore, the bonding strength is not strong enough to survive the long-term reliability test, which is essential for high brightness LED.
U.S. Pat. No. 6,900,473 B2 issued to Shunji Yoshitake et al in 2002 disclosed some manufacturing methods for roughening surface of LED epi-wafers as shown in FIG. 3. Most light generated in the active layer of LED will be trapped inside LED due to the difference of refractive indices between semiconductor LED material and air (or epoxy resin). The roughened LED surface is capable of reducing the total internal reflection between LED surface and air (or epoxy resin), and thus, enhancing the light extraction efficiency of LED. In the embodiments disclosed by Shunji Yoshitake, the roughened LED surface is manufactured either by reducing V/III ratio during the later part of the epitaxial MOCVD growth of upper cladding layer or by thermal etch after epi-growth. However, both methods cannot guarantee the uniformity of the surface roughness. Such limitation makes the implementation of the surface roughening process of AlInGaP light emitting devices difficult.